Interleaving image deposition method

ABSTRACT

A method for depositing an image on a receiving surface utilizing two scans of a print head is provided. Each scan deposits a portion of the image, and the two scans are separated by a non-deposition skip move. The two image portions are joined at a seam to create a composite image. By utilizing identical print head motions along the x-axis direction for all images, this method allows for accurate and repeatable image-half alignment regardless of image width, length or position on the receiving surface.

TECHNICAL FIELD

The present invention relates generally to ink jet printers and, moreparticularly, to a method for printing a composite image comprising twoimage portions that are interleaved at a seam.

BACKGROUND OF THE INVENTION

Ink-jet printing systems commonly utilize either direct printing oroffset printing architecture. In a typical direct printing system, inkis ejected from jets in the print head directly onto the final receivingmedium. In an offset printing system, the print head jets the ink ontoan intermediate transfer surface, such as a liquid layer on a drum. Thefinal receiving medium is then brought into contact with theintermediate transfer surface and the ink image is transferred and fusedor fixed to the medium.

In some direct and offset printing systems, the print head movesrelative to the final receiving medium or the intermediate transfersurface in two dimensions as the print head jets are fired. Typically,the print head is translated along an X-axis while the final receivingmedium/intermediate transfer surface is moved along a Y-axis. In thismanner, the print head "scans" over the print medium and forms adot-matrix image by selectively depositing ink drops at specificlocations on the medium.

With reference now to the image deposition process in an offset printingarchitecture, the print head moves in an X-axis direction that isparallel to the intermediate transfer surface as the drum supporting thesurface is rotated. Typically, the print head includes multiple jetsconfigured in a linear array to print a set of scan lines on theintermediate transfer surface with each drum rotation. Precise placementof the scan lines is necessary to meet image resolution requirements andto avoid producing undesired printing artifacts, such as banding andstreaking. Accordingly, the X-axis (head translation) and Y-axis (drumrotation) motions must be carefully coordinated with the firing of thejets to insure proper scan line placement.

As the size of the desired image increases, the X-axis movement/headtranslation and/or Y-axis motion requirements become greater. Onetechnique for printing larger-format images is disclosed in co-pendingapplication Ser. No. 08/509,844 for INTERLEAVED INTERLACED IMAGING,assigned to the assignee of the present application. This applicationdiscloses a method for interleaving or stitching together multiple imageportions to form a larger composite image. Each of the image portions isdeposited with a separate X-axis translation of the print head. Afterthe deposition of each image portion, the print head is moved withoutfiring the jets to the start position for the next image portion.Adjacent image portions overlap and are interleaved at a seam to formthe composite image.

In this image deposition method, the relative position of each imageportion must be carefully controlled to avoid visible artifacts at theseam joining adjacent image portions. With specific regard to the X-axismovement of the print head, it is necessary to precisely deposit eachimage portion such that adjacent image portions are aligned to properlyinterleave at the seam. Furthermore, the X-axis movement must be capableof repeatably producing composite images having different sizes andpositions on the print medium without creating a visible artifact at theseam between adjacent image portions. Thus, an accurate X-axispositioning mechanism and corresponding positioning method are required.

Prior art ink jet printers have utilized various mechanisms to impartX-axis movement to a print head. An exemplary patent directed to anX-axis positioning mechanism is U.S. Pat. No. 5,488,396 for PRINTERPRINT HEAD POSITIONING APPARATUS AND METHOD (the '396 patent), assignedto the assignee of the present application. This patent discloses amotion mechanism comprising a stepper motor that is coupled by a metalband to a lever arm. Rotation of the lever arm imparts lateral X-axismotion to a positioning shaft that is attached to the print head. Thismechanism translates each step of the stepper motor into one pixel oflateral X-axis movement of the print head. The amount of X-axistranslation per step of the stepper motor is adjustable by aneccentrically mounted ball that is positionable on the lever arm.

While the positioning mechanism of the '396 patent provides highlyaccurate and repeatable positioning of a print head, it is neverthelesssubject to minor displacement errors arising from such factors asimbalances in stepper motor phase and thermal expansion of variouscomponents under changing operating temperatures. Additionally,variations in horizontal jet spacing on the print head can createuncertainty as to the actual X-axis position of a jet, and thusuncertainty in the placement of certain scan lines. Furthermore, whenthe above described method for printing an interleaved composite imageis used, these types of displacement errors are magnified at the seamjoining the two image portions. Even very slight deviations in scan lineplacement on the order of 0.0003 inches (0.0076 mm), normallyimperceptible within a fully interlaced image, generate a visibleartifact due to misalignment at the seam.

Accordingly, the present invention is directed to a print headpositioning method that substantially eliminates displacement errorsarising from mechanical variations in a print head positioningmechanism.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide an image depositionmethod for printing composite images composed of image portionsinterleaved at a seam, the method providing accurate and repeatablealignment of the image portions along the seam.

It is another aspect of the present invention to provide an imagedeposition method in which the print head undergoes identical motionsalong the X-axis for all images, regardless of size or position on thereceiving surface.

It is a feature of the present invention that the same jets on the printhead are used to print the seam for all images, regardless of size.

It is another feature of the present invention that the size andposition of the image are controlled by selectively firing and ignoringjets on the print head.

It is an advantage of the present invention that print artifacts arisingfrom inaccuracies in an X-axis drive mechanism and/or uncertainties injet position on a print head are effectively minimized.

To achieve the foregoing and other aspects, features and advantages, andin accordance with the purposes of the present invention as describedherein, an improved image deposition method for an ink jet printer isprovided. The image deposition method utilizes identical movements ofthe print head along the X-axis to accurately deposit image portionsthat overlap at a seam and form an overall composite image. The samejets on the print head are used to print the seam in all images,regardless of size. By moving the print head identical distances alongthe X-axis for all images, the method of the present inventionsubstantially eliminates visible artifacts at the seam due touncertainties in print head displacement along the X-axis.

Still other aspects of the present invention will become apparent tothose skilled in this art from the following description wherein thereis shown and described a preferred embodiment of this invention, simplyby way of illustration of one of the modes best suited to carry out theinvention. As it will be realized, the invention is capable of otherdifferent embodiments and its several details are capable ofmodifications in various, obvious aspects all without departing from theinvention. Accordingly, the drawings and descriptions will be regardedas illustrative in nature and not as restrictive. And now for a briefdescription of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of an offset ink-jet printingapparatus that utilizes the image deposition method of the presentinvention.

FIG. 2 is a top pictorial view showing the operative relationships amonga stepper motor, first and second pulleys, first and second belts,capstan, split band and print head carriage, these components comprisingan X-axis drive mechanism. This figure also illustrates how the printhead is mounted on the print head carriage to move along the X-axisparallel to the transfer drum.

FIG. 3 is a front isometric view showing how the split band couples thecapstan to the print head carriage.

FIG. 4 is an enlarged elevational view of a portion of the print headface showing parallel vertical columns of ink jets, each column havingfrom top to bottom a cyan, magenta, yellow and black ink jet.

FIG. 5 is a schematic representation of line interlacing with a 4:1interlace ratio and a print head having an interjet spacing N of 10pixels, this figure illustrating the X-axis position of several adjacentink jet columns as viewed from the transfer drum during the printing ofa solid fill image.

FIG. 6 is a schematic representation of a portion of the scan lines thatare printed by the ink jet columns of FIG. 5.

FIG. 7 is an illustration of the scan lines printed at the extreme leftand right edges of an interlaced image where the print head has anintedjet spacing of 10 pixels and the interlace ratio is 4:1.

FIG. 8 is a simplified illustration of an interlaced image having a headand a tail.

FIG. 9 is a simplified illustration of interleaving two image portionsat a seam by aligning the tail of one image portion with the head of theother image portion to form a composite image.

FIG. 10 is a partial elevational view of the face of the print headshowing the leftmost column of ink jets being positioned a distance Afrom the inside face of the left side frame of the printer, thisposition being the point at which printing begins in the method of thepresent invention.

FIG. 11 is a schematic representation showing the movement of the printhead along the X-axis during successive scans according to the method ofthe present invention.

FIG. 12 is an illustration of the position along the X-axis of the seamin a first composite image.

FIG. 13 is an illustration of the X-axis position of the seam in asecond composite image having a different width than the first compositeimage of FIG. 12, the seam in FIG. 13 being located the same distance Calong the X-axis from the inside face of the printer frame as the seamin FIG. 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a schematic illustration of an offset ink-jet printingapparatus 10 that utilizes the image deposition method of the presentinvention. An example of this type of offset printer is disclosed inU.S. Pat. No. 5,389,958 (the '958 patent) entitled IMAGING PROCESS andassigned to the assignee of the present application. The '958 patent ishereby specifically incorporated by reference in pertinent part. Thefollowing description of a preferred embodiment of the method of thepresent invention refers to its use in this type of printing apparatus.It will be appreciated, however, that the method of the presentinvention may be used with various other ink-jet printing apparatus thatutilize different architectures, such as direct printing apparatus inwhich ink is jetted directly onto a receiving medium. Accordingly, thefollowing description will be regarded as merely illustrative of oneembodiment of the present invention.

With continued reference to FIG. 1, the printing apparatus 10 receivesimaging data from a data source 12. A printer driver 14 within theprinter 10 processes the imaging data and controls the operation ofprint engine 16. The printer driver 14 feeds formatted imaging data to aprint head 18 and controls the movement of the print head by sendingcontrol data to a motor controller 23 that activates an X-axis drivemechanism 20. The driver 14 also controls the rotation of the transferdrum 26 by providing control data to a motor controller 22 thatactivates the drum motor 24.

With reference now to FIG. 2, in operation the print head 18 is movedparallel to the transfer drum 26 along an X-axis as the drum 26 isrotated and the print head jets (not shown) are fired. As shown in FIG.3, rotation of the drum 26 creates motion in a Y-axis direction relativeto the print head 18, as indicated by the action arrow Y. In thismanner, an ink image is deposited on an intermediate transfer layer (notshown) that is supported by the outer surface of the drum 26. A moredetailed explanation of an exemplary ink image deposition procedureutilizing line interlacing is provided below. When the image is fullydeposited on the intermediate transfer layer, a final receiving medium,such as a sheet of paper or a transparency, is brought into contact withthe transfer drum 26, and the deposited image is simultaneouslytransferred and fixed (transfixed) to the medium.

FIGS. 2 and 3 illustrate a preferred embodiment of the X-axis drivemechanism 20. A stepper motor 30 is coupled to a first pulley 32 by afirst belt 34. The preferred stepper motor 30 is a bipolar, microstepdriven motor having a displacement of 1.8 degrees per step. The firstpulley 32 is coupled by a second belt 36 to a second pulley 38. As bestseen in FIG. 3, a capstan 40 is coaxially mounted with the second pulley38. A split band 42 couples the capstan 40 to a print head carriage 44.Preferably, the capstan 40 is made from a material having a coefficientof thermal expansion of 1.62×10⁻⁶ /° C. or less. In this manner, thesize of the capstan 40 remains substantially unchanged over a range ofoperating temperatures to facilitate a nearly constant relationshipbetween rotation of the stepper motor 30 and translation of the printhead carriage 44.

As shown in FIG. 3, the band 42 has an elongated shape that includes afirst arm 46, a center portion 48 and a second arm 50 that includes aslot 52 through which the first arm 46 can freely pass. Preferably, theband is punched from stainless steel and has a thickness of 0.008 inches(0.203 mm). The band 42 is firmly attached to the capstan 40 by afastener 54. The second arm 50 wraps about one-half turn around thecapstan 40, extends along the print head carriage 44 and is fastened tothe carriage by a fastener 58. Similarly, the first arm 46 wraps aboutone-half turn around the capstan 40, extends through the slot 52 andalong the print head carriage 44 and is affixed to the carriage by afastener 60.

With reference now to FIG. 2, the print head 18 is mounted on the printhead carriage 44. The print head carriage 44 is slideably mounted on ashaft 64 for movement in the X-axis direction, as indicated by theaction arrow X. In operation, the stepper motor 30 is moved in thedirection of action arrow A, which rotates the first pulley 32 in thedirection of action arrow B. Second pulley 38 and capstan 40 are therebyrotated in the direction of action arrow C, which translates the printhead carriage 44 and the attached print head 18 in the X-axis direction.In the preferred embodiment, this two-stage reduction drive mechanism 20translates each 1.8 degree step of the stepper motor 30 into one-halfpixel of movement in the X-axis direction at the print head carriage 44.Thus, two steps of the motor 30 are required for one pixel of X-axismovement. It will be appreciated by those skilled in the art that othermechanisms for translating the print head 18 in the X-axis direction maybe utilized with the method of the present invention as described below.

With continued reference to FIG. 2, the print head 18 includes a face 70that extends parallel to the transfer drum 26. The drum 26 rotates abouta shaft 74 in the direction of action arrow E. As the drum rotates andthe print head 18 moves along the X-axis, a plurality of ink jets (notshown) on the face 70 eject ink onto the intermediate transfer layer(not shown) on the drum 26. As illustrated in FIG. 6 and explained inmore detail below, one rotation of the transfer drum 26 and asimultaneous translation of the print head 18 along the X-axis whilefiring the ink jets 46 results in the deposition of a diagonal scanline, such as scan line 102, on the intermediate transfer layer of thedrum 26. It will be appreciated that one scan line has a width of onepixel (one pixel width). In 300 dots per inch (dpi) (118 dots per cm.)printing, one pixel has a width of 0.00333 inches (0.085 mm). Thus, thewidth of one 300 dpi scan line equals 0.00333 inches.

FIG. 4 illustrates a portion of the face 70 of the print head 18 asviewed from the intermediate transfer layer of the drum 26. Parallelvertical columns comprising four ink jets 46 each are located across theface 70. While only four columns 82, 84, 86 and 88 are shown, it will beappreciated that the preferred print head 18 utilizes at least 88columns of ink jets 46. Each column of jets 46 includes from top tobottom a cyan C, magenta M, yellow Y and black K ink jet 46. In thismanner, individual ink droplets from a single column of ink jets 46 mayoverlay each other during a scan of the print head 18 to produce adesired color. In the preferred embodiment of print head 18, the blackink jet K in each column of jets 46 is offset two pixels to the left, asviewed in FIG. 4, from the other three ink jets Y, M and C above.

In the preferred embodiment of the method of the present invention,described in more detail below, a line interlacing technique is used tocreate an ink image on the transfer drum 26. Line interlacing entailsprinting adjacent scan lines with two or more different columns of inkjets 46. For example, without reference to an illustration, in a threeto one (3:1) interlace, scan lines 1, 4, 7, etc. are printed with afirst column of jets, lines 2, 5, 8, etc. are printed with a secondcolumn of jets and lines 3, 6, 9, etc. are printed with a third columnof jets. A discussion of line interlacing is presented in co-pendingapplication Ser. No. 08/509,844 for INTERLEAVED INTERLACED IMAGING,assigned to the assignee of the present application. The '844application is hereby incorporated by reference in pertinent part. Thepreferred embodiment of the method of the present invention utilizes 300dpi printing. It will be appreciated that the method of the presentinvention may also be practiced with other printing resolutions, such as600 dpi.

With continued reference to FIG. 4, adjacent columns of ink jets 46 arespaced apart along the X-axis by an intedjet spacing of N pixel widths.The intedjet spacing N determines the number of adjacent scan lines thatmust be printed to produce a solid fill image. As a single scan linecorresponds to one rotation of the transfer drum 26 and a simultaneousmovement or step of the print head 18 along the X-axis, the interietspacing N also dictates the number of rotations of the drum that mustoccur to create a solid fill image. It follows that a print head 18having an interjet spacing of N=10 requires 10 rotations of the transferdrum to produce a solid fill image. It will be appreciated that the sizeof a solid fill image is dependent upon the size of the print head andthe line interlacing ratio that is utilized.

As explained above, a scan line is printed by rotating the transfer drum26 while simultaneously moving the print head 18 in the X-axis directionand firing the ink jets 46. To create the above-described 3:1 interlace,the print head 18 moves or steps a distance n of three pixels in theX-axis direction for every rotation of the transfer drum. In practice,the X-axis drive mechanism 20 moves the print head 18 at a constantvelocity while the transfer drum 26 rotates. Thus, an alternativerepresentation of a 3:1 interlace pattern is the numeric step sequencethrough which the print head 18 travels to print a solid fill image. Fora print head 18 having an interjet spacing N=10 and using a 3:1interlace, the X-axis step sequence ofthe print head is 3, 3, 3, 3, 3,3, 3, 3, 3, 3.

FIGS. 5 and 6 schematically illustrate an example of the lineinterlacing produced by a print head 18 having an interjet spacing N=10and an image deposition procedure that utilizes a 4:1 interlace ratio(n=4). FIG. 5 is a "time-lapse" illustration of the X-axis position ofseveral ink jet columns as viewed from the drum 26 during the printingof a solid fill image. Alternatively expressed, FIG. 5 is a schematicrepresentation of a portion of a solid fill image showing the positionof the ink jets 46 prior to each of the ten rotations of the drum 26.Each of the adjacent symbols in the row 50 represents one column of jets46 on the print head 18 as viewed from the drum 26. All of the symbolshaving the same shape and orientation represent the same column of jets46 of FIG. 4. For purposes of understanding, it may be easier for thereader to visualize each symbol as a single ink jet 46 corresponding toa particular jet column that is positioned at the beginning of arotation of the drum 26. The number within each symbol designates aparticular rotation of the transfer drum 26, with 1 being the firstrotation and 10 being the last rotation among the 10 rotations requiredto print a solid fill image. It follows that symbols 82-1, 84-1, 86-1and 88-1 represent the X-axis position of jet columns 82, 84, 86 and 88,respectively, at the beginning of the first rotation of the drum 26.

Symbols 82-1 through 82-10 represent the position of jet column 82 atthe beginning of the first through the tenth rotations of the drum 26,respectively. Similarly, symbols 84-1 through 84-10 represent theposition of jet column 84 at the beginning of the first through thetenth rotations of the drum 26, respectively. This illustrates that foran interlace of 4:1, the print head 18 moves a distance of n=4 pixelsalong the X-axis with each rotation of the drum 26. From FIG. 4 it willbe noted that jet column 82 is adjacent to jet column 84. It followsthat in FIG. 5 the distance between symbol 82-1 and symbol 84-1 is equalto the interjet distance or spacing of N=10 pixels. FIG. 5 alsoillustrates that a 4:1 interlaced image is comprised of bands 71 thatcontain output from four different jet columns. Each of the bands 71 hasa width equal to the interjet spacing N=10.

FIG. 6 illustrates a portion of the scan lines that are printed by thejet columns represented by the symbols in FIG. 5. Scan lines 102, 103and 104 are each printed by jet column 82 on consecutive rotations ofthe drum 26. More specifically, scan line 102 is printed on the firstrotation, scan line 103 is printed on the second rotation and scan line104 is printed on the third rotation. Each symbol along the upper row 90represents the position of a particular jet column at the beginning of ascan line, i.e., the beginning of a rotation of the drum 26. Thisposition also corresponds to the end of the previous scan line printedby this jet column on the previous rotation of the drum 26, as indicatedby the same symbol with the same rotation number on the lower row 92.Alternatively expressed, each symbol along the lower row 92 representsthe position of a particular jet column at the end of a scan line. Forexample, symbol 82-2, corresponding to jet column 82, is simultaneouslypositioned at the end of scan line 102 (on the lower row 92) and thebeginning of scan line 103 (on the upper row 90). The one exception tothis positioning occurs at the beginning of the first rotation of thedrum 26, as represented by symbols 82-1 and 84-1. As this is the firstrotation of the drum 26, there are no previous scan lines that have beenprinted. Thus, symbols 82-1 and 84-1 represent only the beginning ofscan lines 102 and 107, respectively, and these symbols do not have acounterpart on row 92.

With continued reference to FIG. 6, the print head step pattern for theillustrated 4:1 interlace is 4, 4, 4, 4, 5, 4, 4, 4, 4, 4. All of thesteps cover the same distance along the X-axis of 4 pixels per drumrevolution except for the 5th step, which is one greater than theothers. This extra pixel of movement on the fifth rotation of the drum26 is termed an adjustment move for the purposes of this application.The adjustment move is illustrated in the printing of scan lines 100 and105. For example, at the end of the printing of scan line 100, jetcolumn 80 is positioned at the beginning of scan line 102, this positionbeing indicated in phantom by the symbol 80-6 in the lower row 92.However, scan line 102 was printed by jet column 82 on the firstrotation of the drum 26. Thus, were printing to continue with jet column80 in this position, the scan line printed by jet column 80 wouldoverlap scan line 102 upon the next drum rotation, and the interlacewould have a gap of one scan line.

To avoid this problem, the print head 18 of FIG. 2 is advanced one extrapixel (the adjustment move) at the end of the fifth rotation of the drum26. In this manner, jet column 80 is properly positioned adjacent to thebeginning of scan line 102, this position being indicated by the symbol80-6'. This adjustment move must occur when the jets are not printing onthe drum 26. In the preferred embodiment, the surface area of the drum26 is larger than the largest image to be printed on the drum 26. Thisallows the drum 26 to include a dead band portion over which the printhead jets do not fire. The adjustment move occurs within this dead bandportion of the drum 26.

With continued reference to FIG. 6, at the end of ten rotations of thedrum 26, the interlaced image is complete and printing stops. This isindicated by the symbols 94 and 96 at the end of scan lines 108 and 109,respectively.

The above example of a 4:1 interlace utilized a print head 18 having aninterjet spacing N of 10. This relatively small value of N wasintentionally chosen to facilitate the schematic illustration of theinterlace pattern. In the preferred embodiment of the present invention,the print head 18 utilizes an interjet spacing N of 28 with a 4:1interlace. The corresponding step sequence of the preferred print head18 is 4, 4, 4, 4, 4, 4, 6, 4, 4, 4, 4, 4, 4, 5.35, 4, 4, 4, 4, 4, 4, 6,4, 4, 4,4,4, 4, 4. An adjustment move occurs at the end of the 7th, 14thand 21st rotation of the drum 26. The middle adjustment move at the 14throtation is given a non-integer value of 1.35 pixels to utilize scanline pairing to further improve image quality. Scan line pairing altersthe middle adjustment move in an interlace pattern to a value slightlydifferent from an integer scan line distance. In this manner, allsubsequently printed scan lines will not be equidistant from both oftheir adjacent scan lines, but rather moved closer or "paired" with oneof their adjacent scan lines. Scan line pairing of interlaced images isaddressed in more detail in copending application Ser. No. 08/381,615entitled "PAIRING OF INK DROPS ON A PRINT MEDIUM" (the '615application), filed on Jan. 30, 1995 and assigned to the assignee of thepresent application. The '615 application is hereby incorporated inpertinent part.

Care must be taken when printing the edges of an interlaced image whenthe interlace ratio is greater than 2:1. FIG. 7 illustrates the resultsof printing using all of the jet columns on print head 28. Row A is theextreme left edge of an interlaced image and row B is the extreme rightedge of an interlaced image where the interjet distance N is 10 and theinterlace ratio is 4:1. It will be appreciated that rows A and B areactually printed along the same horizontal axis, and FIG. 7 shows A andB vertically separated for illustration purposes only. Each rectangle inrows A and B represents a scan line of the image, with the numbers abovethe rectangles representing the particular jet column that prints thatscan line. As drawn, scan lines with a "1" are the printed by theleftmost or first jet column, scan lines with a "2" are printed by thesecond jet column, and so forth. Similarly, scan lines with an "88" areprinted by the rightmost or last jet column, scan lines with an "87" areprinted by the next adjacent jet column, and so forth.

With reference to row A, the scan lines 100, 101, 102, and 103 areprinted during the first rotation of the drum 26, the scan lines 104,105, 106, and 107 are printed during the second rotation, and so forth.It will be appreciated that the majority of the interlaced image extendsto the right of the scan lines illustrated in row A. As this figureillustrates, the resulting interlaced image has gaps 110 and is notfully filled until scan line 112. The print region containing gaps 110at the beginning of printing is termed a "head" 120 for the purpose ofexplanation in this application. Gaps 110 in head 120 would normally beconsidered to be unacceptable in a printed image. To avoid the gaps, theleftmost edge of the interlaced image may be mapped to scan line 112such that no scan lines to the left of scan line 112 are printed.

Referring now to row B, a corresponding situation occurs at the otherend of print head 18, resulting in a "tail" 130. To avoid gaps 132 intail 130, the rightmost edge of the interlaced image may be mapped toscan line 134 such that no scan lines to the right of scan line 134 areprinted.

FIG. 8 illustrates a simplified view of an interlaced image 140 alongthe X-axis, showing the fully filled region 142. The head 120 and tail130 are simply represented as ramps. By confining the output of printhead 28 to between scan lines 148 and 150, the gaps can be avoided andonly the fully filled region 142 will be printed.

Alternatively, with reference back to FIG. 7, it can be seen that thegaps 110 of the head 120 are perfectly aligned with the printed scanlines in the tail 130. It follows that a composite interlaced image maybe interleaved or "stitched" together from two image portions byaligning their respective tail 130 and head 120. FIG. 9 shows asimplified representation of interleaving two image portions at a seam165 to result in a wider composite interlaced image 167. Printing acomposite interlaced image 167 substantially wider than print head 18can be achieved by printing a first image portion 160 having a tail 162,moving print head 18 to the beginning 163 of the tail 162, and thenprinting a second image portion 164 having a head 166 that interleaveswith tail 162. In the present application, printing the first imageportion 160 and tail 162 is referred to as "Scan 1", moving the printhead 18 to the beginning 163 of the tail 162 is referred to as the "skipmove" and printing the head 166 and the second image portion 164 isreferred to as "Scan 2."

The resulting composite image 167 can be mapped between a first scanline 168 in the first image portion 160 and a second scan line 170 inthe second image portion 164. The composite image 167 need not be thefull width available between the first and second scan lines 168 and170, respectively. Alternatively expressed, the jet columns on the printhead 18 that print the extreme left and right edges of the image 167 arenot required to be adjacent to one end of the print head. Thus, if thewidth of the desired composite interlaced image 167 is less than twicethe maximum width of a single image portion printable by print head 18,then less than all of the jet columns will be used to print the firstand second image portions 160 and 164, respectively. Furthermore, itwill be recognized that a composite interlaced image can comprise morethan two image portions.

From the above description of interlacing a first and second imageportion 160, 164, it will be appreciated that the relative position ofeach image portion along the X-axis must be carefully controlled toinsure proper alignment at the seam 165. It is also desirable to varythe width of one or both image portions 160, 164 to allow for printingcomposite images 167 having variable widths.

One method for varying the width of the composite image along the X-axisis to vary the distance of the skip move between the end of Scan 1 andthe beginning of Scan 2. However, varying the distance of the skip moverequires varying the jet columns that print the head 162 and tail 166that comprise the seam 165. As mentioned above, even very slightinaccuracies in X-axis displacement will result in noticeable artifactsat the seam 165. Thus, if there is even 0.0003 inch uncertainty in thedisplacement of the print head 18 along the X-axis, varying the ink jetsthat print the seam 165 will result in a visible artifact at the seam.

To avoid artifacts at the seam produced by X-axis uncertainties, themethod of the present invention utilizes identical motions of the printhead 18 along the X-axis, including an identical skip distance, for allcomposite images of various widths. Alternatively expressed, thebeginning and ending positions of the print head 18 along the X-axis forScan 1 and Scan 2 are identical for all images. The width of thecomposite image is controlled by changing the jets 46 that print theextreme left and right scan lines 168, 170 of the image 167 (FIG. 9). Asexplained in more detail below, this method allows the same jets toprint the seam 165 in all images, thereby avoiding uncertainties in theX-axis position of the print head 18 at the seam 165.

With reference now to FIG. 10, the method of the present inventionbegins with positioning the print head 18 a distance A from a fixedreference point. In the preferred embodiment, the fixed reference pointis the inside face 202 of the left side frame 204 of the printer 10 asviewed from the drum 26. The distance A is preferably 1.98 inches (5.03cm) as measured from the centerline of the leftmost jet column 1.

From this position the print head 18 is moved a short distance towardthe left side frame 204 of the printer 10, as indicated by the phantomoutline of the left end 19 of the print head in FIG. 10. In thepreferred embodiment this distance is 8 pixels. This allows the X-axisdrive mechanism 20 to accelerate the print head 18 to a constantvelocity before Scan 1 begins printing at the desired distance A fromthe reference point. FIG. 11 schematically illustrates the position ofthe print head 18 along the X-axis at the beginning of Scan 1 (18-0),end of Scan 1 (18-1), beginning of Scan 2 (18-2) and end of Scan 2(18-3). As mentioned above, the preferred image deposition methodutilizes a 4:1 interlace and an interjet spacing(g N of 28. Accordingly,to complete Scan 1 the drum 26 will make 28 revolutions and the printhead will advance along the X-axis according to the step pattern of 4,4, 4, 4, 4, 4, 6, 4, 4, 4, 4, 4, 4, 5.35, 4, 4, 4, 4, 4, 4, 6, 4, 4, 4,4, 4, 4, 4. Thus, in 300 dpi printing, the total distance S travelled bythe print head 18 along the X-axis to complete Scan 1 is 117.35 pixels,where one 300 dpi pixel=0.0333 inches (0.8466 mm).

At the end of Scan 1, the print head 18 prints the tail 162 of the firstimage portion 160 (FIG. 9). As explained above, the preferred print head18 utilizes 88 columns of ink jets 46, the columns being numbered 1-88from left to right as viewed from the drum 26 (see FIG. 10). In thepreferred embodiment of the method of the present invention, jet columns72-74 are utilized to print the tail 162 of the first image portion 160.It follows that jet columns 75-88 are not used to print during Scan 1.As mentioned above, in an important aspect of the present invention, thesame jet columns 72-74 are used to print the tail 162 of the first imageportion 160 in all composite images 167, regardless of width. The widthof the composite image 167 is controlled strictly by selective jetaddressing.

Upon the completion of Scan 1 and the tail 162, the print head 18executes a skip move and advances to the beginning 163 of the tail 162,which is also the beginning of the head 166 (FIG. 9). This positioncorresponds to the beginning of Scan 2, as indicated by the referencenumeral 18-2 in FIG. 11. The skip move is coordinated with the rotationof the drum 26 to occur over an integral number of drum revolutions, andthe ink jets are not fired during the skip move. The distance Btravelled by the print head 18 during the skip move is fixed for allcomposite images, regardless of width. The formula for calculating theskip distance B is as follows: B=NW-{ (H-1)+1-(n-1)!N+S}, whereN=interjet spacing; W=maximum solid fill imaging distance along theX-axis that is addressable by the print head, expressed as the number ofjet columns spanning this distance; H=number of jet columns on the printhead; n=distance advanced by print head with each drum rotation; andS=distance advanced by print head to print the first image portion. Inthe preferred embodiment, B=1394.65 pixels in 300 dpi printing, or 4.644inches (11.81 cm).

Scan 2 begins with the printing of the head 166 of the second imageportion 164. As explained above, the head 166 interleaves with the tail162 to form the seam 165 that joins the first and second image portions160, 164 (FIG. 9). In the preferred embodiment, jet columns 18-20 of theprint head 18 are utilized to print the head 166 of the second imageportion 164. It follows that jet columns 1-17 are not used for printingduring Scan 2. As with Scan 1, these same jet columns 18-20 are used toprint the head 166 of the second image portion 164 in all compositeimages, regardless of width. Additionally, Scan 2 advances the printhead 18 the same distance S along the X-axis, 117.35 pixels, andutilizes the same step pattern as Scan 1. At the completion of Scan 2,the complete composite image 167 is formed. It will be appreciated thatthe method of the present invention contemplates printing and combiningmore than two image portions to form a composite image.

To summarize an important aspect of the present invention, the printhead 18 undergoes identical movements along the X-axis to print allimages, regardless of image width, length or position on the receivingsurface. More specifically, with respect to a fixed reference point onthe printer frame, the position of the print head at the beginning ofScan 1 and the distance travelled by the print head in executing Scan 1,the skip move and Scan 2 are all the same for every image printed. Thewidth, length and position of a particular composite image arecontrolled by selectively firing the ink jets 46 on the print head 18.Alternatively expressed, the width, length and position of an image arecontrolled by varying the ink jets 46 that print the left and rightedges of the image 167, as represented by the scan lines 168 and 170 inFIG. 9. The method of the present invention advantageously allows thesame ink jets 46 to print the seam 165 joining the first and secondimage portions 160, 164 in all composite images. It also follows thatthe position of the seam 165 along the X-axis relative to the fixedreference point is fixed for all images, regardless of their width,length or position on the media. As shown in FIGS. 12 and 13, the seam165 is positioned the same distance C along the X-axis from the fixedreference point represented by the inside face 202 of the left sideframe 204 of the printer 10. The difference in the placement of the seam165 within the image 167 versus the placement of the seam 165 within theimage 167' results from variations in media position on the drum 26 andvariations in image size and position on the media.

As explained above, even very slight inaccuracies in X-axis displacementwill result in noticeable artifacts at the seam 165. Accordingly, thestep pattern followed by the print head 18 along the X-axis during Scans1 and 2 along with the skip move must be precisely controlled by theX-axis drive mechanism 20. In an ideal X-axis drive mechanism 20,rotational commands to the stepper motor 30 would correspond linearly totranslation of the print head along the X-axis (FIG. 2). However, inpractice, several factors related to the components of the X-axis drivemechanism 20 cause X-axis displacement errors. These factors includeimbalances in phases of the stepper motor 30, eccentricities in thefirst and second pulleys 32, 38 and the capstan 40 and imperfect toothengagement between the belts 34, 36 and the pulleys.

To compensate for inaccuracies in the X-axis drive mechanism 20 of aparticular printer 10, the velocity of the print head along the X-axisis adjusted by a calibration factor to coordinate print headdisplacement with drum rotation. Alternatively expressed, for a givenX-axis drive mechanism 20, the velocity of the print head 18 along theX-axis during Scan 1 is adjusted by a first calibration factor toachieve the desired print head step pattern. In a similar manner, thevelocity of the print head 18 along the X-axis during Scan 2 is adjustedby a second calibration factor, and the velocity of the print head 18during the skip move is adjusted by a skip calibration factor. In thepreferred embodiment, each calibration factor is determined empiricallyby analyzing the deviation between the X-axis displacement commanded bythe printer driver 14 (FIG. 1) and the actual X-axis displacement. Inthis manner, the actual print head displacement along the X-axis iscalibrated to match the required displacements of the method of thepresent invention.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed. The terms and expressions which have been employed in theforegoing specification are used therein as terms of description and notof limitation. The use of such terms and expressions is not intended toexclude equivalents of the features shown and described or portionsthereof. Many changes, modifications, and variations in the materialsand arrangement of parts can be made, and the invention may be utilizedwith various different printing apparatus, all without departing fromthe inventive concepts disclosed herein.

The preferred embodiment was chosen and described to provide the bestillustration of the principles of the invention and its practicalapplication to thereby enable one of ordinary skill in the art toutilize the invention in various embodiments and with variousmodifications as is suited to the particular use contemplated. All suchmodifications and variations are within the scope of the invention asdetermined by the appended claims when interpreted in accordance withbreadth to which they are fairly, legally, and equitably entitled. Allpatents cited herein are incorporated by reference in their entirety.

What is claimed is:
 1. In an ink-jet printer having a print head and areceiving surface that undergo relative movement, the print headincluding a plurality of jets for ejecting ink, the plurality of jetsbeing spaced apart by a distance of 28 pixel widths along an X-axis, amethod for printing multiple composite images having different widths,each of the composite images having at least two image portionsinterleaved at a seam, the method comprising the steps of:(a)positioning the print head a distance A along the X-axis from a fixedreference point; (b) printing a set of scan lines of a first imageportion by moving the print head relative to the receiving surface adistance of 4 pixel widths along the X-axis while ejecting ink from atleast a portion of the plurality of jets; (c) repeating step (b) 27times until the first image portion having a tail is completed,including moving the print head an additional distance of 2 pixel widthsafter printing a 7^(th) set of scan lines, 1.35 pixel widths afterprinting a 14^(th) set of scan lines and 2 pixel widths after printing a21^(st) set of scan lines; (d) moving the print head relative to thereceiving surface along the X-axis a fixed skip distance B; (e) printinga set of scan lines of a second image portion by moving the print headrelative to the receiving surface a distance of 4 pixel widths along theX-axis while ejecting ink from a portion of the plurality of jets; (f)repeating step (e) 27 times until the second image portion having a headis completed, including moving the print head an additional distance of2 pixel widths after printing a 7^(th) set of scan lines, 1.35 pixelwidths after printing a 14^(th) set of scan lines and 2 pixel widthsafter printing a 21^(th) set of scan lines, where the tail of the firstimage portion interleaves with the head of the second image portion toform the seam and to complete at least a portion of a first compositeimage having a width; (g) repeating steps (a) through (f) to print atleast a portion of a second composite image having a different widththan the first composite image, where movement of the print headrelative to the receiving surface along the X-axis is identical forprinting the first composite image and the second composite image. 2.The method of claim 1, wherein a width of the first composite image anda width of the second composite image are controlled by selectivelyfiring the plurality of jets of the print head.
 3. The method of claim1, wherein the same jets of the print head are used to print the seam inthe first composite image and the second composite image.
 4. The methodof claim 1, wherein the seam is spaced from the reference point by thesame distance in the first composite image and the second compositeimage.
 5. The method of claim 1, wherein a velocity of the print headalong the X-axis is adjusted by a first calibration factor whileprinting the first image portion.
 6. The method of claim 1, wherein avelocity of the print head along the X-axis is adjusted by a secondcalibration factor while printing the second image portion.
 7. Themethod of claim 1, wherein the fixed skip distance B is defined by theformula B=NW-{ (H-1)+1-(n-1)!N+S}, where N=are the distance between jetsalong the X-axis: W=a width along the X-axis of a maximum solid fillimage that is addressable by the print head; H=a total number of theplurality of jets; n=a distance advanced by the print head with eachdrum rotation; and S=a distance advanced by the print head to print thefirst image portion.
 8. The method of claim 1, wherein the fixed skipdistance B is 1394.65 pixel widths in 300 dpi printing.
 9. The method ofclaim 1, wherein the fixed skip distance B is adjusted by a skipcalibration factor.
 10. The method of claim 1, wherein step (c)comprises repeating step (b) N-1 times to complete the first imageportion having a tail.
 11. The method of claim 10, wherein step (f)comprises repeating step (e) N-1 times to complete the second imageportion having a head.
 12. The method of claim 1, wherein no ink isejected from the plurality of jets during step (d).